Solar cell and method and apparatus for manufacturing solar cell

ABSTRACT

A thin solar cell is provided, a decreased amount of an Al paste used for the solar cell without occurrence of a problem of ball-up which is a defect in appearance. A method of manufacturing such a solar cell as well as a manufacturing apparatus used therefor are provided. This manufacturing method is applicable with substantially no change in the conventional material and process. The solar cell has an Al paste electrode on the back surface and at least a part of an outer edge of the Al paste is thicker than any remaining part.

This application is a divisional of U.S. patent application Ser. No.10/335,954 filed Jan. 3, 2003, now U.S. Pat. No. 7,381,887 which claimspriority under 35 U.S.C. §119(a) on Japanese Application 2002-013226filed Jan. 22, 2002, all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to solar cells, particularly a solar cellwith its back surface having a paste electrode of Al thereon, andrelates to methods and apparatuses for manufacturing the solar cell.

2. Description of the Background Art

FIG. 9 shows a structure of a solar cell with its back surface providedwith a paste electrode made of Al. The structure of the solar cell ishereinafter described in connection with a manufacturing process shownin FIG. 10. For a crystalline silicon-based cell, after a p-type siliconsubstrate 1 is etched, an n-type diffusion layer 2 is deposited on oneside of the substrate that serves as a photo-receiving plane and ananti-reflection film 3 is formed thereon in order to decrease thesurface reflectance. On the other side of the substrate that is oppositeto the photo-receiving plane (the above-mentioned other side is hereinreferred to as “back surface” as appropriate), a paste of Al isscreen-printed, dried at approximately 150° C. and thereafter fired inthe air at approximately 700° C. to form a paste electrode 4 of Al.Moreover, a silver paste is screen-printed on some parts of the backsurface and the photo-receiving plane according to a pattern, dried andthereafter fired in an oxidizing atmosphere at a high temperature toform paste electrodes 5 and 6 of silver. The resultant device isimmersed in a flux, silver paste electrodes 5 and 6 are thensolder-coated, and the device is rinsed and dried to produce the solarcell. The solar cell generally has a size of, for example, 10 cm, 12.5cm or 15.5 cm per side.

In the screen-printing process, a cell to be subjected to printing isfixed on a stage, and a screen mask is lowered to adjust the distancebetween the cell and the screen mask to an appropriate one. The Al pasteis supplied onto the screen mask and a squeegee is moved whilepressurizing the paste so as to transfer the Al paste onto the cellthrough the screen mask.

FIG. 11A is a plan view of a p-type silicon substrate 11 showing an Alpaste electrode 14 formed on the back surface of the substrate. FIG. 11Bshows a cross section of the substrate along line a-a in FIG. 11A Thethickness of the Al paste which has been dried is 45 to 55 μm, theaverage thickness being approximately 50 μm.

In order to manufacture solar cells excellent in long-term reliabilityat low cost, there has recently emerged a need for decrease of theamount of the Al paste used in the process that constitutes aconsiderable part of a solar cell. In addition, as it is known thatdecrease of the thickness of the Al paste electrode is effective forlessening any warp of the solar cell and, in this sense, it is urgentlyrequired to decrease the amount of the Al paste.

The decrease of the amount of the Al paste and the decrease of thethickness after drying to 40 μm or less for example, are unsatisfactoryfor the following reason. Referring to FIG. 12, after the Al paste isfired, ball-shaped Al particles 19 of a diameter ranging from severaltens of μm to several hundred μm are generated on the outer edge of theelectrode. This trouble called ball-up causes a problem that the cellcannot be commercialized due to the defect in appearance.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a thin solar cellproduced with a decreased amount of an Al paste, without the trouble ofball-up. Another object of the present invention is to provide a methodof manufacturing such a solar cell as discussed above as well as anapparatus used therefor.

A solar cell of the present invention has an Al paste electrode on itsback surface, and at least a part of an outer edge of the Al pasteelectrode has a greater thickness than that of any remaining part of theAl paste electrode. Preferably, only one side, only two sides oppositeto each other, or only two sides adjacent to each other, of the outeredge of the Al paste electrode is/are thicker than any remaining part ofthe Al paste electrode. It is also preferable that the thickness of theAl paste electrode successively changes.

According to a method of manufacturing a solar cell of the presentinvention, screen printing is performed by pressurizing an Al pastethrough a screen mask to transfer the Al paste to the back surface ofthe solar cell. The distance between the screen mask and the backsurface of the solar cell in the screen printing is changed depending ona part of the back surface that is to be printed, and a part moredistant from the screen mask is printed with a thicker Al paste while apart less distant from the screen mask is printed with a thinner Alpaste.

The distance between the screen mask and the back surface of the solarcell may be changed by providing a spacer to at least a part of a spacebetween a frame of the screen mask and a mask holder, by providing aspacer to at least a part of a region to be screen-printed that islocated along and outside the perimeter of the solar cell, by providinga spacer to at least a part between the solar cell and a stage forsecuring the solar cell thereon, or by inclining the stage for securingthe solar cell thereon.

Accordingly, the solar cell of the present invention may be manufacturedby a screen printer having a stage for securing the solar cell thereonthat is inclined to change the distance between the screen mask and theback surface of the solar cell. The solar cell of the present inventionmay also be manufactured by moving a squeegee faster above a part of theback surface of the solar cell that is to be printed with a thicker Alpaste. Moreover, the solar cell of the present invention may bemanufactured by performing printing of the Al paste at least oncebefore, after or before and after the screen printing for a part of theback surface of the solar cell that is to be applied with a thicker Alpaste.

Further, the solar cell of the present invention may be manufactured byperforming spray coating of the Al paste at least once before, after orbefore and after the screen printing for a part of the back surface ofthe solar cell that is to be applied with a thicker Al paste. The solarcell of the present invention may be manufactured by using a screen maskhaving a pressed part used for applying a thinner Al paste. Themanufacturing apparatus of the present invention thus includes thescreen mask with a pressed part for applying a thinner Al paste.

A solar cell of the present invention may be manufactured by using ascreen mask with the distance between an edge of a pattern of the screenmask and a frame of the screen mask that is closest to the edge being atmost 30 mm, preferably at most 20 mm. The manufacturing apparatus of thepresent invention thus includes the screen mask with the distance fromthe frame being at most 30 mm, preferably 20 mm.

The solar cell of the present invention may be manufactured by using asqueegee applying printing pressure which is changed depending on a partto be printed, and accordingly a thicker Al paste is formed on a partapplied with a lower printing pressure relative to a part applied with ahigher printing pressure. The manufacturing apparatus of the presentinvention includes a squeegee applying a decreased printing pressure bya part of an edge of the squeegee which is shorter than a remaining partof the edge of the squeegee. Further, the manufacturing apparatus of thepresent invention includes a squeegee having a part of an edge andanother part thereof that are attached at respective angles differentfrom each other to apply a lower printing pressure by the part of theedge than that applied by that another part.

According to the present invention, a thin solar cell can be providedthat uses a decreased amount of an Al paste without being accompanied bya problem of ball-up which is a defect in appearance, as well as amethod of manufacturing the solar cell and an apparatus used therefor.Moreover, according to the present invention, the solar cell can beproduced without substantially changing the conventional materials andprocess. In addition, the present invention is effective in that thecell cracks less frequently since the outer edge of the aluminumelectrode has an increased thickness.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E each show a cross sectional view of a solar cellaccording to the present invention.

FIGS. 2 to 4 each show a cross sectional view of a screen printer usedfor the present invention.

FIG. 5 is a cross sectional view of a screen mask according to thepresent invention.

FIG. 6 is a plan view of a screen mask according to the presentinvention.

FIG. 7A is a front view of a squeegee according to the present inventionand FIG. 7B is a left side view thereof.

FIG. 8A is a front view of a squeegee according to the present inventionand FIG. 8B is a left side view thereof.

FIG. 9 is a cross sectional view showing a structure of a solar cellhaving an Al paste electrode formed on the back surface thereof.

FIG. 10 is a flowchart illustrating a process of manufacturing the solarcell having the Al paste electrode formed on the back surface thereof.

FIG. 11A is a plan view of a p-type silicon substrate having an Al pasteelectrode formed thereon and FIG. 11B is a cross sectional view alongline a-a in FIG. 11A.

FIG. 12 is a cross sectional view of a p-type silicon substrate with anAl paste electrode formed thereon where the trouble of ball-up occurs.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Solar Cell>

The present invention has a characteristic that, at least a part of anouter edge of an Al paste electrode formed on the back surface of asolar cell has a greater thickness than that of any remaining part ofthe electrode.

The outer edge of the Al paste electrode has a thickness which is madegreater than that of any remaining part, since the ball-up tends tooccur on the outer edge of the Al paste electrode and the increasedthickness of the electrode makes the ball-up less prone to occur.Specifically, the ball-up is less prone to occur when the thickness ofthe electrode is at least 45 μm, while the ball-up is more prone tooccur when the thickness is 40 μm or less. In view of this, even if theAl paste electrode has a thickness of 40 μm or less in order to reducethe amount of the Al paste used for forming the Al paste electrode, thethickness of the outer edge of the electrode is preferably at least 45μm for preventing the ball-up and the thickness is more preferably atleast 55 μm.

A reason for increasing the thickness of at least a part of the outeredge of the Al paste electrode is that, the ball-up does not uniformlyoccur on the electrode. In other words, the probability of occurrence ofthe ball-up is higher on a side of the electrode that enters a firingfurnace first, while the probability thereof is lower on a side of theelectrode that enters the fixing furnace last. Moreover, inconsideration of the challenge to decrease the amount of Al paste to beused, any part where the ball-up tends to occur is preferably madethicker.

Specifically, as shown in FIGS. 1A to 1C, only one side of the outeredge of an Al paste electrode 14 that is put into a firing furnace firstis preferably larger in thickness. In this case, preferably thethickness is gradually varied as shown in FIG. 1A according to anymethod of varying the thickness of the printed electrode for example.Alternatively, only two sides including the side entering the furnacelast are preferably made thicker as shown in FIG. 1D. Further, as theprobability of occurrence of the ball-up is higher on the outer edge ofthe Al paste electrode while the ball-up rarely occurs on the centralpart of the electrode, all of the four sides constituting the outer edgeof the electrode are made thick as shown in FIG. 1E. It is to be notedhere that the electrode is not always put into the furnace in thedirection perpendicular to opposite two sides of the electrode, or theelectrode may be polygonal in shape. In these cases, preferably only twosides adjacent to each other of the outer edge of the Al paste electrodeare made thick (not shown).

<Manufacturing Method>

The solar cell of the present invention has the characteristic that theouter edge of the Al paste electrode is relatively thick. The thicknessof the electrode may be varied from part to part of the electrode by,for example, changing the distance between the screen mask and the backsurface of the solar cell, changing any conditions of the printing speedor pressure for example, changing the number of times the printing isdone, changing the specification of the screen mask, or changing thespecification of the squeegee.

A method of manufacturing a solar cell according to the presentinvention has a characteristic that, the distance between the screenmask and the back surface of the solar cell undergoing screen printingis varied according to a part of the electrode to be printed so that apart of the electrode at a relatively long distance from the screen maskhas a thicker Al paste printed thereon while a part at a relativelyshort distance therefrom has a thinner Al paste printed thereon. Whenthere is a longer distance between the screen mask and the back surfaceof the solar cell, a greater amount of an Al paste is allowed to passthrough the screen mask and thus a greater amount of the Al paste can betransferred onto the back surface of the solar cell to print the Alpaste of a greater thickness. The relation between the distance from thescreen mask to the back surface of the solar cell and the thickness ofthe Al paste varies depending on the viscosity, for example, of the Alpaste to be used. If an Al paste may be printed to have a thickness ofapproximately 40 μm with the distance of 0.3 to 0.6 mm between thescreen mask and the back surface of the solar cell, the thickness ofapproximately 45 μm of the paste is achieved by providing a distance of1.5 to 2.0 mm between the screen mask and the back surface of the solarcell.

Preferred methods of varying the distance between the screen mask andthe back surface of the solar cell according to a part to be printed arenow described. Referring to FIG. 2, a spacer 27 may be provided betweena frame 24 of a screen mask 23 and a mask holder 22, a spacer 28 may beprovided to at least a part of the perimeter of a region of a cell 25(hereinafter referred to as “solar cell” as appropriate) that is to bescreen-printed, a spacer 29 may be inserted into at least a part of theregion between a stage 26 holding cell 25 and cell 25, or stage 26holding cell 25 may be inclined. Any of the above-discussed methodsallows the screen mask 23 which is moved down for printing to beinclined with respect to the back surface of the solar cell, and thusthe distance between the back surface of the solar cell and the screenmask is varied according to a part to be printed.

Another method of manufacturing a solar cell according to the presentinvention as shown in FIG. 3 has a characteristic that a squeegee 31 isspeedily moved above a part 38 where an Al paste is to be printed to agreater thickness in screen-printing. The squeegee is moved faster andaccordingly, the screen mask separates more speedily from the backsurface of the solar cell as the squeegee passes. Then, a greater amountof an Al paste is passed through the screen mask and transferred ontothe back surface of the solar cell and the part is printed with athicker paste relative to any part above which the squeegee movesslower. The squeegee moves at a speed which is different depending onthe specification, for example, of a screen printer. If the squeegee canbe moved at a speed of 30 to 50 mm/sec to print an Al paste to athickness of approximately 40 μm, a thickness of approximately 45 μm isachieved by moving the squeegee at a higher speed of 120 to 200 mm/sec.

Still another method of manufacturing a solar cell according to thepresent invention has a characteristic that, any part to be printed witha thicker Al paste is subjected to at least one printing operation ofthe Al paste before, after, or before and after screen printing. Thesame part is printed a plurality of times to generate a thicker printedpaste relative to other parts. One example of this method is shown inFIG. 4, according to which a printer is controlled as follows. Afterscreen printing and before returning of a squeegee 41 with a scraper(ink turner) 48 provided thereon, a new cell 45 is fixed on a stage 46and, in the course of returning of squeegee 41 and scraper 48 back tothe original position, scraper 48 is lowered onto a part 49 to beprinted with a thicker paste to pressurize a screen mask 43. By thismethod, pre-printing can be performed by scraper 48 before screenprinting by squeegee 41, which is preferable in that the pre-printing isperformed with easy manipulation after the screen printing. Scraper 48may be lowered by 1.0 mm or less to print an Al paste of 5 to 20 μm inthickness, which may be different depending on the specification of theAl paste. In addition, the Al paste used for the first printing can beused for the second and subsequent printing operations. If an Al pastecontains a glass component, an Al paste of a different kind may be used.

A further method of manufacturing a solar cell according to the presentinvention has a characteristic that, a part where a thicker Al paste isto be printed undergoes at least one spray-coating of an Al pastebefore, after, or before and after screen printing. The spray coatingallows that part to be printed with a thicker paste relative to otherparts. Although another coating method except for the spray coating maybe employed, the spray coating is a simplest one and thus preferable.When the spray coating is made, such an organic solvent as n-butylcarbitol acetate is preferably added to the Al paste for decreasing theviscosity of the paste relative to the viscosity for the screenprinting. Moreover, the Al paste used for the first printing can be usedfor the second and subsequent printing operations. If an Al pastecontains a glass component, an Al paste of a different kind may be used.

The solar cell of the present invention may be manufactured by a screenmask as shown in FIG. 5. Specifically, screen mask 53 has a pressedsection 53 a corresponding to a part 51 for printing a thin Al paste.The pressed section 53 a is thinner than a non-pressed screen masksection 53 b. Accordingly, the pressed section of the increased densitypasses less Al paste, resulting in a thinner Al paste in screenprinting. Regarding the pressing work, calendar roll may be used forpressurizing.

A further method of manufacturing a solar cell according to the presentinvention has a characteristic as shown in FIG. 6. Specifically, inorder to provide a section 63 a of a screen mask for printing a thickerAl paste and a section 63 b for printing a thinner Al paste, thedistance X between a pattern edge 60 and the closest frame 64 of thescreen mask is set to 30 mm or less. As this distance X is 30 mm orless, the speed at which the screen mask separates from the back surfaceof the solar cell by the tension of the screen mask increases when thescreen mask is pressurized by a squeegee. Then, the amount of the Alpaste passed through the screen mask to be transferred onto the backsurface of the solar cell increases and accordingly the thicker Al pastecan be printed, as achieved by increasing the speed of movement of thesqueegee. With a shorter distance X, a thicker paste can be printed andthus the distance is preferably 20 mm or shorter.

A further method of manufacturing a solar cell according to the presentinvention has a characteristic that the pressure of the squeegee inscreen printing is different depending on the part to be printed. Alower printing pressure results in a smaller amount of an AL paste whichis wiped off and thus results in a thicker printed Al paste. Therelation between the printing pressure and the amount of printed Alpaste is different depending on the type of the Al paste and the speedof movement of the squeegee, for example. If a printing pressure of 5.0to 6.0 kg/cm produces an amount of the printed paste of 35 to 40 μm, theprinting pressure may be reduced to 2.0 to 2.5 kg/cm to produce theprinted paste of 40 to 45 μm. The printing pressure may be varied by thesqueegee through the following methods for example. Referring to FIGS.7A and 7B, a certain part 71 a of the edge of a squeegee 71 may be cutoff to become shorter than another part 71 b of the edge so as todecrease the printing pressure. Alternatively, as shown in FIGS. 8A and8B, a certain part 81 a and another part 81 b of the edge of a squeegee81 may be attached at different angles respectively so as to decreasethe printing pressure of the edge part 81 a relative to that of the edgepart 81 b of squeegee 81.

<Manufacturing Apparatus>

A screen printer according to the present invention has a characteristicthat a stage on which a solar cell is secured is inclined to vary thedistance between the screen mask and the back surface of the solar cellaccording to the part to be printed.

A screen mask according to the present invention has a characteristicthat a part of the screen mask that is used for printing a thin Al pasteis pressed. Further, a screen mask of the present invention has acharacteristic that the distance between the pattern edge of the screenmask and the closest mask frame is 30 mm or less, and preferably 20 mmor less.

A squeegee according to the present invention has a characteristic thata part of the edge of the squeegee is made shorter than another part ofthe edge in order to decrease the printing pressure of the shorter edgepart. Further, a squeegee of the present invention has a characteristicthat a certain part of the edge of the squeegee and another part thereofare attached at different angles respectively so as to decrease theprinting pressure of that certain part relative to the printing pressureof that another part.

The above-described screen printer, screen masks and squeegees areappropriate for an apparatus for manufacturing the solar cell asdescribed above that has the characteristic that at least a part of anouter edge of an Al paste electrode has a greater thickness than that ofany remaining part of the electrode.

EXAMPLE 1

A p-type silicon substrate in the shape of a square of 125 mm×125 mmwith a thickness of 330 microns was texture-etched. On one side of thesubstrate, an n-type diffusion layer having a sheet resistance ofapproximately 50 ω was formed at 900° C. through thermal diffusion of P.On the diffusion layer, a silicon nitride film with a thickness ofapproximately 60 nm was formed through plasma CVD so as to serve as ananti-reflection film. Then, on the back surface of the substrate, apaste of Al was screen-printed, dried at 150° C. and put into an IRfurnace with the thickest part entering the furnace first. The paste wasfired at 700° C. in the air to produce a paste electrode of Al.Moreover, silver pastes were respectively screen-printed on the backside and the light-receiving plane in accordance with a pattern, dried,and thereafter fired at 600° C. for 2 minutes in an oxidizing atmosphereto produce paste electrodes of silver. Finally, the silver pasteelectrodes were each coated with a solder layer and accordingly, thesolar cell was completed.

For the screen printing, a printer SS150 manufactured by Seishin TradingCo., Ltd. was used, and 3718G1 manufactured by MURATA MFG. CO., LTD. wasused as the Al paste. Further, as shown in FIG. 2, using screen mask 23having a thickness of 150 μm and formed of a mesh made of SUS150 and aframe having a size of 320 mm×320 mm, spacer 27 is inserted into oneside of frame 24 of the screen mask and secured to mask holder 22, andthen printing was done. (Spacers 28 and 29 were not provided in thisExample.) Spacer 27 used here was a plastic plate having a width of 15mm, a length of 150 mm and a thickness of 1 mm. The inserted spacer 27inclined screen mask 23 with respect to solar cell 25 in lowering a maskholder 22 and screen printing.

The resultant Al paste electrode had a cross section as shown in FIG.1A, with its thickness successively changing from the thickest parttoward the thinnest part. After drying and before firing, the thickestpart and the thinnest part of the Al paste had respective thicknesses of45 μm and 39 μm, with the average thickness of 42 μm. A conventional Alpaste is 45 to 55 μm in thickness with the average thereof beingapproximately 50 μm. Then, the amount of the Al paste used in Example 1was smaller by 16% as a whole relative to the conventional paste, andthus a thin solar cell was achieved. It is noted that the ball-up didnot occur after firing. This example accordingly produced the solar cellhaving electric characteristics and reliability comparable to those ofconventional products, through a simple operation of inserting thespacer, by means of conventional materials and process.

EXAMPLE 2

A solar cell was manufactured as done in Example 1 except that spacer 27shown in FIG. 2 is removed and spacer 28 made of plastic and having awidth of 15 mm, a length of 150 mm and a thickness of 1 mm was attachedto stage 26 along one side of cell 25 to perform screen printing.

A resultant Al paste electrode had a cross section as shown in FIG. 1A,with its thickness successively changing from the thickest part towardthe thinnest part. After drying and before firing, the thickest part andthe thinnest part of the Al paste had respective thicknesses of 45 μmand 40 μm, with the average thickness of 42 μm. Then, the amount of theAl paste used in Example 2 was smaller by 16% as a whole relative to theconventional paste. It is noted that the ball-up did not occur afterfiring the Al paste. This example accordingly produced the solar cellhaving electric characteristics and reliability comparable to those ofconventional products, through a simple operation of attaching thespacer, by means of conventional materials and process.

EXAMPLE 3

A solar cell was manufactured as done in Example 1 except that, on thetop surface of stage 26 shown in FIG. 2, an adhesive cellophane tape 29of 150 mm in length was affixed, the thickness of the cellophane tape 29was adjusted to 150 μm, and thereafter cell 25 was fastened thereon toperform screen printing (spacer 27 was removed here).

A resultant Al paste electrode had a cross section as shown in FIG. 1A,with its thickness successively changing from the thickest part towardthe thinnest part. After drying and before firing, the thickest part andthe thinnest part of the Al paste had respective thicknesses of 46 μmand 40 μm, with the average thickness of 43 μm. Then, the amount of theAl paste used in Example 3 was smaller by 14% as a whole relative to theconventional paste. It is noted that the ball-up did not occur afterfiring the Al paste. This example accordingly produced the solar cellhaving electric characteristics and reliability comparable to those ofconventional products, through a simple operation of affixing thespacer, by means of conventional materials and process.

EXAMPLE 4

As shown in FIG. 3, a solar cell was manufactured as done in Example 1except that screen printing was done with squeegee 31 moved at a highspeed of 160 mm/sec above the part 38 and moved at a speed of 40 mm/secequal to that of Example 1 above the remaining part (with spacer 27removed here).

A resultant Al paste electrode had a cross section as shown in FIG. 1B.After drying and before firing, the thickest part and the thinnest partof the Al paste had respective thicknesses of 45 μm and 41 μm, with theaverage thickness of 42 μm. Then, the amount of the Al paste used inExample 4 was smaller by 16% as a whole relative to the conventionalpaste. It is noted that the ball-up did not occur after firing the Alpaste. This example accordingly produced the solar cell having electriccharacteristics and reliability comparable to those of conventionalproducts, through a simple operation of increasing the speed of movingthe squeegee, by means of conventional materials and process.

EXAMPLE 5

A solar cell was manufactured as done in Example 1 except that screenprinting was done with squeegee 31 moved at a high speed of 160 mm/secabove the parts 38 and 39 and moved at a speed of 40 mm/sec equal tothat of Example 1 above the remaining part (with spacer 27 removedhere), as shown in FIG. 3.

A resultant Al paste electrode had a cross section with two protrudingparts (not shown). After drying and before firing, the thickest part andthe thinnest part of the Al paste had respective thicknesses of 45 μmand 41 μm, with the average thickness of 43 μm. Then, the amount of theAl paste used in Example 5 was smaller by 14% as a whole relative to theconventional paste. It is noted that the ball-up did not occur afterfiring the Al paste. This example accordingly produced the solar cellhaving electric characteristics and reliability comparable to those ofconventional products, through a simple operation of increasing thespeed of moving the squeegee, by means of conventional materials andprocess.

EXAMPLE 6

After Example 1 was carried out, a cell was newly fastened onto thestage. Spacer 27 was removed, scraper 48 was thereafter placed oversqueegee 41 as shown in FIG. 4, and then the scale of the printer wasadjusted to lower scraper 48 at the position above the part 49 by 0.5 mmrelative to other parts so as to start printing. When scraper 48 withsqueegee 41 was moved back to scrape the Al paste, scraper 48 at theposition above the part 49 is lowered by 0.5 mm to cause the Al paste tobe printed on that part 49. Then, as Example 1, squeegee 41 was moved tocarry out screen printing.

The part 49 was printed twice. Accordingly, after drying and beforefiring, the thickest part 49 and the thinnest part of the Al paste hadrespective thicknesses of 45 μm and 41 μm, with the average thickness of42 μm. Then, the amount of the Al paste used in Example 6 was smaller by16% as a whole relative to the conventional paste. It is noted that theball-up did not occur after firing the Al paste. This exampleaccordingly produced the solar cell having electric characteristics andreliability comparable to those of conventional products, through asimple operation of lowering the scraper by 0.5 mm, by means ofconventional materials and process.

EXAMPLE 7

As shown in FIG. 2, a solar cell was manufactured as done in Example 1except that screws (not shown) for fastening stage 26 were adjusted tolower the left side of stage 26 by 0.5 mm and then secure the stage asit is so as to perform screen printing (without spacer 27).

Accordingly, after drying and before firing, the thickest part and thethinnest part of the Al paste had respective thicknesses of 48 μm and 40μm, with the average thickness of 42 μm. Then, the amount of the Alpaste used in Example 7 was smaller by 16% as a whole relative to theconventional paste. It is noted that the ball-up did not occur afterfiring the Al paste. This example accordingly produced the solar cellhaving electric characteristics and reliability comparable to those ofconventional products, through a simple operation of changing the angleof placement of the stage, by means of conventional materials andprocess.

EXAMPLE 8

As shown in FIG. 5, the part of screen mask 53 that corresponds to theregion 51 was pressed by a calendar roll. Consequently, the non-pressedpart 53 b of screen mask 53 was 70 μm in thickness while the pressedpart 53 a was 59 μm in thickness. A solar cell was manufactured as donein Example 1 except that this screen mask was used to perform screenprinting (without spacer 27).

Accordingly, after drying and before firing, the thickest part and thethinnest part of the Al paste had respective thicknesses of 45 μM and 39μm, with the average thickness of 42 μm. Then, the amount of the Alpaste used in Example 8 was smaller by 16% as a whole relative to theconventional paste. It is noted that the ball-up did not occur afterfiring the Al paste. This example accordingly produced the solar cellhaving electric characteristics and reliability comparable to those ofconventional products, through a simple operation of pressing the screenmask, by means of conventional materials and process.

EXAMPLE 9

As shown in FIG. 6, in order to produce the part 63 a and the part 63 bof screen mask 63 respectively for printing the Al paste to larger andsmaller thicknesses, the distance X between the pattern edge 60 and theclosest frame 64 of the screen mask was adjusted to 20 mm. In addition,since frame 64 was of small size, an adapter (not shown) was used forprinting. A solar cell was then manufactured as done in Example 1 exceptfor the above-discussed procedure (without spacer 27).

After drying and before firing, the thickest part and the thinnest partof the Al paste had respective thicknesses of 45 μm and 42 μm, with theaverage thickness of 43 μm. Then, the amount of the Al paste used inExample 9 was smaller by 14% as a whole relative to the conventionalpaste. It is noted that the ball-up did not occur after firing the Alpaste. This example accordingly produced the solar cell having electriccharacteristics and reliability comparable to those of conventionalproducts, through a simple operation of changing the specification ofthe screen mask, by means of conventional materials and process.

EXAMPLE 10

A solar cell was manufactured as done in Example 1 except that an Alpaste was uniformly screen-printed on the entire back surface of thep-type silicon substrate to a thickness, after being dried, of 36 μm,the paste was then dried, and one side corresponding to the outer edgeof the resultant Al paste electrode was subjected to screen printingagain (without spacer 27).

The resultant Al paste electrode had a cross section as shown in FIG.1C. The thickest part and the thinnest part of the Al paste hadrespective thicknesses of 55 μm and 36 μm, with the average thickness of40 μm. Then, the amount of the Al paste used in Example 10 was smallerby 20% as a whole relative to the conventional paste. It is noted thatthe ball-up did not occur after firing the Al paste. This exampleaccordingly produced the solar cell having electric characteristics andreliability comparable to those of conventional products, through asimple operation of performing screen printing twice, by means ofconventional materials and process.

EXAMPLE 11

A solar cell was manufactured as done in Example 1 except that an Alpaste was uniformly screen-printed on the entire back surface of thep-type silicon substrate to a thickness, after being dried, of 36 μm,the paste was then dried, and two opposing sides corresponding to theouter edge of the resultant Al paste electrode were subjected to screenprinting again (without spacer 27).

The resultant Al paste electrode had a cross section as shown in FIG.1D. The thickest part and the thinnest part of the Al paste hadrespective thicknesses of 55 μm and 36 μm, with the average thickness of41 μm. Then, the amount of the Al paste used in Example 11 was smallerby 18% as a whole relative to the conventional paste. It is noted thatthe ball-up did not occur after firing the Al paste. This exampleaccordingly produced the solar cell having electric characteristics andreliability comparable to those of conventional products, through asimple operation of performing screen printing twice, by means ofconventional materials and process.

EXAMPLE 12

An Al paste was uniformly screen-printed on the entire back surface ofthe p-type silicon substrate to a thickness, after being dried, of 36μm, the paste was then dried, and one side was spray-coated, that oneside being a place where the outer edge of the resultant Al pasteelectrode was to be located. For the spray coating, 20% by mass ofn-butyl carbitol acetate was added to the Al paste which was used forthe screen printing in order to decrease the viscosity of the paste. Anair gun was used for the spray coating to form a spray coating of 20 mmin width. A solar cell was accordingly produced as done in Example 1except for the above-described process (no spacer 27 was attached).

The resultant Al paste electrode had a cross section as shown in FIG.1C. The thickest part and the thinnest part of the Al paste hadrespective thicknesses of 53 μm and 36 μm, with the average thickness of40 μm. Then, the amount of the Al paste used in Example 12 was smallerby 20% as a whole relative to the conventional paste. It is noted thatthe ball-up did not occur after firing the Al paste. This exampleaccordingly produced the solar cell having electric characteristics andreliability comparable to those of conventional products, through asimple operation of adding one spray coating process, by means ofconventional materials and process.

EXAMPLE 13

An Al paste was uniformly screen-printed on the entire back surface ofthe p-type silicon substrate to a thickness, after being dried, of 36μm, the paste was then dried, and four sides were spray-coated, thatfour sides being a place where the outer edge of the resultant Al pasteelectrode was to be located. For the spray coating, the Al paste(diluted) used in Example 12 was employed. An air gun was used for thespray coating to form a spray coating of 20 mm in width. A solar cellwas accordingly produced as done in Example 1 except for theabove-described process (no spacer 27 was attached).

The resultant Al paste electrode had a cross section as shown in FIG.1E. The thickest part and the thinnest part of the Al paste hadrespective thicknesses of 53 μm and 36 μm, with the average thickness of42 μm. Then, the amount of the Al paste used in Example 13 was smallerby 16% as a whole relative to the conventional paste. It is noted thatthe ball-up did not occur after firing the Al paste. This exampleaccordingly produced the solar cell having electric characteristics andreliability comparable to those of conventional products, through asimple operation of adding one spray coating process, by means ofconventional materials and process.

EXAMPLE 14

As shown in FIG. 7A, a certain part 71 a of the edge of squeegee 71 thatis a side contacting the screen mask was cut off over 100 μm and 20 mmin width. FIG. 7B shows a left side of this squeegee 71. A solar cellwas accordingly produced as done in Example 1 except for the use of thesqueegee (no spacer 27 was attached).

The thickest part and the thinnest part of the Al paste had respectivethicknesses of 45 μm and 41 μm, with the average thickness of 42 μm.Then, thex amount of the Al paste used in Example 14 was smaller by 16%as a whole relative to the conventional paste. It is noted that theball-up did not occur after firing the Al paste. This exampleaccordingly produced the solar cell having electric characteristics andreliability comparable to those of conventional products, through asimple operation of cutting off the edge of the squeegee, by means ofconventional materials and process.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A method of manufacturing a solar cell, screen printing being performed by pressurizing an Al paste through a screen mask to transfer the Al paste to the back surface of said solar cell, the distance between the screen mask and the back surface of said solar cell in the screen printing being changed depending on a part of said back surface that is to be printed, and a part more distant from the screen mask being printed with a thicker Al paste while a part less distant from the screen mask being printed with a thinner Al paste.
 2. The method of manufacturing a solar cell according to claim 1, wherein a spacer is provided to at least a part of a space between a frame of the screen mask and a mask holder in said screen printing to change the distance, in printing, between the screen mask and the back surface of said solar cell depending on a part of said back surface that is to be printed.
 3. The method of manufacturing a solar cell according to claim 1, wherein a spacer is provided to at least a part of a region located along and outside the perimeter of said solar cell in said screen printing to change the distance, in printing, between the screen mask and the back surface of said solar cell depending on a part of said back surface that is to be printed.
 4. The method of manufacturing a solar cell according to claim 1, wherein a spacer is provided to at least a part between said solar cell and a stage for securing said solar cell thereon in said screen printing to change the distance, in printing, between the screen mask and the back surface of said solar cell depending on a part of said back surface that is to be printed.
 5. The method of manufacturing a solar cell according to claim 1, wherein a stage for securing said solar cell thereon is inclined in said screen printing to change the distance, in printing, between the screen mask and the back surface of said solar cell depending on a part of said back surface that is to be printed.
 6. A method of manufacturing a solar cell, according to claim 1, wherein screen printing performed by pressurizing an Al paste by a squeegee being moved to transfer the Al paste to the back surface of said solar cell through a screen mask, and said squeegee being moved faster above a part of the back surface of said solar cell that is to be printed with a thicker Al paste.
 7. A method of manufacturing a solar cell, according to claim 1, wherein printing or spray coating of the Al paste being performed at least once before, after or before and after said screen printing, for a part of the back surface of said solar cell that is to be applied with a thicker Al paste.
 8. A method of manufacturing a solar cell, according to claim 1, wherein the distance between an edge of a pattern of the screen mask that is used for printing the Al paste and a frame of the screen mask that is closest to the edge being at most 30 mm.
 9. A method of manufacturing a solar cell, according to claim 1, wherein screen printing performed by pressurizing an Al paste by a squeegee being moved to transfer the Al paste to the back surface of said solar cell through a screen mask, printing pressure applied by said squeegee being changed depending on a part to be printed, and a thicker Al paste being printed on a part applied with a lower printing pressure relative to a part applied with a higher printing pressure. 